This invention relates generally to nuclear reactors and more particularly, to methods for installing jet pump slip joint labyrinth seals.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser has about 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
Excessive leakage flow can cause oscillating motion in the slip joint, which is a source of detrimental vibration excitation in the jet pump assembly. The slip joint leakage rate can increase due to single loop operation, increased core flow, or jet pump crud deposition. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue.
High levels of flow induced vibration (FIV) are possible under some abnormal operational conditions having increased leakage rates. Reducing flow leakage through the slip joint results in reduced oscillating motion and FIV. A labyrinth seal or labyrinth grooves in jet pump assemblies, as described in U.S. Pat. No. 4,285,770, facilitate reducing flow leakage through the slip joint. Leakage is reduced by the increase in flow resistance resulting from the labyrinth groove geometry flow area expansion and contraction losses.
Jet pump assemblies and other components within nuclear reactor pressure vessels (RPV) become irradiated, and those nearest the reactor core become highly irradiated. Because these components are radioactive, they are kept underwater to provide radiation shielding to workers in the proximity of the reactor components. Cobalt-60, which emits two high-energy gamma rays upon radioactive decay, is generally the isotope with the greatest impact on non-core component radioactivity. Cobalt-60 is produced by a neutron activation reaction with the cobalt present in most austenitic stainless steels used in nuclear reactor component fabrication and in some component hardfacing or surface treatments. Providing labyrinth seals in jet pump assemblies of previously operated, irradiated jet pump assemblies would reduce flow leakage and FIV.
In one aspect, a method for cutting a labyrinth seal in a nuclear reactor jet pump assembly is described. The method includes removing an irradiated jet pump inlet mixer from the jet pump assembly, positioning the jet pump inlet mixer underwater, and cutting at least one circumferential groove in an irradiated inlet mixer outside surface.
In a further aspect, a method for cutting a labyrinth seal in a nuclear reactor jet pump assembly is described. The method includes removing an irradiated jet pump inlet mixer from the jet pump assembly, positioning at least a portion of a cutting system in a jet pump diffuser, and cutting at least one circumferential groove in a jet pump diffuser inner surface.